1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic method, such as a compact copying machine, a printer, a composite machine and the like, more particularly, an image forming apparatus which is improved for solving the inconvenience when using a substantially spherical toner. The present invention also relates to a process cartridge which is attachable to and detachable from the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus utilizing an electrophotographic method, image forming is performed by forming an electrostatic latent image on the surface of an image holding member composed of an organic photosensitive material or the like in the form of a drum or a belt by the known electrophotographic process, developing the electrostatic latent image into a toner image using a toner, electrostatically transferring this toner image onto a recording paper directly or via an intermediate transfer material, and coalescing the toner on the surface of the recording paper by heating or the like.
As such a toner, there is mainly used a dry toner obtained by dispersing a colorant, a charge control agent and the like, if necessary, in a resin as a main component, and forming the dispersion into a particle shape. Such a dry toner, regardless of it is to be used in a one-component developer or a two-component developer, is prepared, in most cases, by a so-called mechanical grinding method of kneading and uniformly dispersing a colorant and the like in a resin as a main component, mechanically grinding the dispersion, and classifying the ground material so as to obtain a desired particle diameter and particle size distribution.
Meanwhile, in such an image forming apparatus, due to recent demands for high image quality, it has been required that a particle size of the toner be smaller, and a particle size distribution of such a toner be narrower. When the particle size distribution is wide, a ratio of toner particles having a relatively smaller particle size and toner particles having a relatively larger particle size increases, and this increase may lead to the following inconveniences. When there are many toners having a small particle size, these toners become easy to fly from a developing unit, resulting in pollution of an image forming apparatus. Moreover, in the case of a two-component developer, since the small size toner becomes easy to adhere to a carrier, the chargeability of a toner is reduced. On the other hand, when there are many toners having a large particle diameter, there arises the inconvenience such as reduction in the image quality.
However, when one produces such a toner of a small particle size having a narrow particle size distribution by the aforementioned mechanical grinding method, the productivity and the yield become considerably worse, resulting in a cost increase. For this reason, a wet process such as a polymerization method, a dissolution method, or the like is proposed as a process for preparing such a toner.
The polymerization method is to obtain toner particles by granulation through a polymerization reaction in the state where a colorant, etc. is mixed with a monomer material. Since a particle size can be controlled by adjusting reaction time or the like, a particle size distribution can be very narrow in principle.
The dissolution method is to obtain toner particles by preparing an oily phase by dissolving or dispersing a binding resin, a colorant and the like in an organic solvent, and suspension-granulating the oily phase components in an aqueous phase, and this process can make a particle diameter small and control a particle size distribution.
A toner obtained by a wet process such as a polymerization method or a dissolution method has the characteristics that a particle shape is substantially spherical, while a toner particle obtained by the aforementioned mechanical grinding method generally does not have a specific shape. For this reason, since a substantially spherical toner obtained by a wet process has a smaller particle diameter and a spherical shape as compared with a toner obtained by the mechanical grinding method, a contact area between the toner and the surface of an image holding member is small and, therefore, the toner is known to have merits that a force of adhering the toner to the surface of an image holding member becomes small, and the transferring efficiency is considerably improved. Since such a better transferring efficiency reduces the number of toners which are discarded without being utilized for forming an image, an amount of a toner to be used can be reduced, and thus such a toner is economical, and environmentally friendly as compared with a conventional toner.